This invention relates to an electrical monitor circuit for monitoring a status of a three-phase current limiting device. The current limiting device is intended to limit or interrupt a current in a three-phase line. Consequently, the current limiting device comprises three one-phase current limiting means, one for each one-phase line.
The monitor circuit according to the invention is intended to monitor whether the current limiting device is in its normal conducting, i.e. relatively low resistance status or whether there is any current limiting or current interrupting effect of the current limiting device present.
A prior art solution concerning monitoring of a current limiting device is shown in DE 43 40 632 A1. Therein, the voltage across a PTC resistor being connected in series with a load break switch is used to trigger the opening of the load break switch. The PTC resistor is intended to improve the current limiting capability of the load break switch which finally interrupts the current.
What is also known are means to optically signalize that a thermal fuse has blown. These conventional proposals mainly relate to circuits in automobiles and are shown e.g. in U.S. Pat. Nos. 4,281,322, 4,514,723 and EP-A 0 911 853.
D 41 18 346 shows a monitor circuit for monitoring fuses in a three-phase line. This monitor circuit comprises two star points, each star point being connected to three respective taps on one respective side of the fuses and each star point relating to one side. In order to detect failure of all three fuses, additional circuitry between one of the star points and the corresponding side of the fuses is provided.
Also DE 42 13 443 shows a monitor circuit using taps on each side of fuses in a three-phase line. The circuitry connected to each one of both sides of the fuses is described as a triangle circuit, respectively.
Finally, U.S. Pat. No. 5,530,613 describes series connections of PTC-polymer current limiters and contactors controlled by control circuits responsive to the voltage across the PTC-polymer current limiters. Alternatively, series connections of PTC-polymer current limiters, contactors and circuit breakers are mentioned.
The technical problem underlying the present invention is to provide a simple electrical monitor circuit for monitoring a status of a three-phase current limiting device with improved efficiency.
According to the invention, this technical problem is solved by means of an electric monitor circuit for monitoring a status of a three-phase current limiting device for a three-phase line comprising three one-phase current limiting means, that circuit comprising: six taps, one respective tap for a first and a second side of each current limiting means, said taps being divided into two groups of three taps each; two star points, said three taps in each group being connected to one respective star point for each group via a respective resistor; and a voltage detector for detecting a voltage between said star points and producing a signal responsive to said voltage difference, characterized in that two taps on a first side of said current limiting device and one tap on a second side of said current limiting device are connected to one of said star points.
Further the invention relates to an apparatus for interrupting an electrical current in an electrical line, comprising an electrical monitor circuit as defined above, said three-phase current limiting device, a switch in said three-phase line and in series connection with said three-phase current limiting device, wherein said monitor circuit is adapted to, in case of a resistance change of at least one of said single-phase current limiting means, detect said voltage for a predetermined time period after said change and produce said signal only if said resistance change is present for said time period, to open said switch.
Still further, the invention relates to a circuit for power supplying and protecting an electrical motor comprising a monitor circuit or an interrupting apparatus as defined above.
One idea of the invention is that in a three-phase circuit it is generally not important in case of tripping of at least one of three one-phase current limiting means, to know which or how many current limiting means have tripped. To the contrary, it simplifies the monitor circuit and the structure of the signals to be produced by the monitor circuit, if there is only one signal indicating the tripping of anyone of three one-phase current limiting means. I.e. the status of the three-phase current limiting device is differentiated between a normal status in which ail current limiting means are normally conducting, and a triggered status in which one of them is triggered.
Therefore, the invention provides a voltage measurement between two star points, each of which combines three respective taps. These in total six taps are distributed on the three single phase current limiting means, one tap on each side of each means. Thereby, triggering of one of said means leads to an increase of the voltage between both star points and can thus be detected and used to produce a signal indicating the triggered status.
According to the invention, one star point is a combination of two taps of one side and one tap of the other side of said three-phase current limiting device, the other star point being a combination of the remaining three taps, again two on the latter and one on the former side. The advantage of the circuit design is that the monitor circuit is responsive equally to triggering of anyone, any two or all three single phase current limiting means and that the circuit design is remarkably simple.
If all resistors connecting the taps with their respective star point have equal values within each group, the voltage detected will be substantially zero in the normal conducting state of the current limiting device and different from zero in any other case, i.e. in the triggered status.
The signal produced responsive to the status of the current limiting device can be used for signalizing purposes only, e.g. for controlling a display, lamp etc. or for information of a remote control and supervision center. Preferably however, the monitor circuit according to the invention feeds said signal to a control circuit in order to control a three-phase switch in said monitored line. According to the basic idea of the invention, the switch is opened according to the signal, i.e. independently from which one of the single-phase current limiting means has triggered. In any case of triggering the switch is opened for all three lines. This function is advantageous for any application wherein a remaining one or remaining two phases must not be active in case of the triggering of another phase.
This applies especially to a preferred application of the invention in a circuit for power supplying and protecting an electrical motor. A three-phase motor can be destroyed if operated by only one or two phases. Therefore, the invention provides an important advantage over the mere application of independent single-phase current limiting devices in motor circuits.
The control circuit can be power supplied from the three-phase line. One possible realization is a combination of two taps in the three-phase line with a rectifier circuit. If the security aspect of a galvanic separation is important, an isolating transformer can be used, e.g. on the AC side of the above rectifier circuit.
A suitable choice for the three-phase switch is a load break switch, switch fuse and/or contactor. A regular circuit breaker is usually not necessary because of the limiting effect of the current limiting device. However, also a circuit breaker could be provided for in order to have a safe breaking capability in case of a failure of the current limiting device.
For the measurement it is preferred to introduce a galvanical separation between the control circuit controlling the switch and the starpoints in order to avoid disturbances of the formers potential by the latter. One preferred realisation of such a galvanic separation is an optical coupling, e.g. by a combination of a light emitting diode (LED) and a photo diode. Also, a security aspect can be relevant here.
In order to provide for said response time period, a capacitor and a resistor can be provided within the control circuit This has also proven to be an efficient measure especially for noise occurring during start up of an electrical motor. Reference is made to the following description of the embodiment.
In order to prevent a triggering of the voltage detector by very short disturbances and noise in the three-phase line that do not lead to a limiting effect of the current limiting device, it is also effective to provide for an another capacitor, additionally or alternatively, between both star points. Also the charging delay time of this capacitor can provide for a voltage buildup between the star points over said time period and thus for said time continued detection by the voltage detector.
As mentioned above, the signal produced responsive to the voltage detection according to the invention can be used also for signaling purposes instead of opening a switch. Further, both functions can be combined, e.g. by including a light emitting device such as a LED in the control circuit. Or by fitting the signal to a remote control center besides local action in terms of opening a switch.
So far, the term xe2x80x9ccurrent limiting devicexe2x80x9d has been used in its generality in order to demonstrate that the invention works with the variety of special embodiments for this device. However, the most common and commercially important embodiment is a set of three thermal fuses, each for one phase. A fuse is in fact more than a current limiting device, actually a current interrupting device. However, this is not essential for the invention. Accordingly, in this application the term current limiting also includes a limitation to zero. In case of thermal fuses, it is preferred to open at least one three-phase switch at a high voltage side of the fuses, more preferably two three-phase switches, each on one side of the fuses, it can be guaranteed by opening the switch(es) that the contacts in which the fuses are mounted and in which a replacement has to be effected, are free of any harmful potential.
Another preferred choice for the current limiting device is a set of three PTC resistors, especially PTC-polymer resistors.
PTC resistors are current limiting means in the sense that they are able to limit a current to small or very small values within a very short response time. However, they do not really provide for a galvanic interruption of the current as a thermal fuse does. Therefore, PTC resistors can advantageously be combined with conventional switches, e.g. a switch fuse or load break switch and/or a contactor, in order to improve the response time and thus the limiting capability of the switch and simultaneously provide for a galvanic interruption at least in cases in which this proves to be necessary. Consequently, the invention also relates to an apparatus for interrupting an electrical current with a three-phase current limiting device, especially a three-phase PTC resistor, and a switch in series connection, wherein the triggering of the current limiting device, i.e. a change from the PTC""s low resistance status to its high resistance status, is detected by means of the star point voltage for a certain time period. By choosing appropriate values for the pacitance of the capacitors and the resistance of the resistor, the time period can be predetermined.
In any case, voltage disturbances and noises of short time extension can be xe2x80x9cfiltered offxe2x80x9d and do not lead to an opening of the switch. On the other hand, if the high voltage across the PTC resistor is present over said time period, it can be concluded, that a serious fault has been detected leading to an opening of the switch. For further details of this interrupting apparatus, reference is made to the embodiment of the invention as described below.
The contactor can also be a microrelais switch.
As already mentioned above, the electric monitor circuit according to the inventionxe2x80x94as well as the interrupting apparatus just describedxe2x80x94can be used in a power supplying and protecting circuit for an electrical motor.
In the following, the invention will be described with reference to the figures which show preferred embodiments of the invention.